EDIT: Concerning an INSERT: i don't think it's always the same whether you have an (clustered) index or not. A Clustered index creates an index in which the logical order of the key values determines the physical order of the corresponding rows in a table. Depending on your choice of clustered index this may affect insert performance. Read the introduction to clustered and non-clustered index data structures at sql-server-performance.com.

One or more initialization actions have failed. Resolve these errors before attempting to run the storage emulator again. These errors can occur if SQL Server was installed by someone other than the current user. Please refer to http://go.microsoft.com/fwlink/?LinkID=205140 for more details.

What this error means is:

I was using the useDevelopmentStorage=true parameter, so the emulator connects to the default sqlexpress instance (MACHINE\sqlexpress) on the pc.

Once connected, he tried to create a new database

If SQL Express is not installed by the user your are working with. The current user is NOT a sysadmin and can’t create any databases). Adding yourself is neither a solution or enabling the SA user as you are not granted access to do so.